1. Field of the Invention
This invention relates generally to communications systems having mobile subscribers and, more particularly, to a method for routing calls to such mobile subscribers.
2. History of The Prior Art
In certain communications systems, such as in some radiotelephone systems, the entire service area of a system is divided into a number of local service areas or exchanges. In each local area, radio access units are located at one or more key points to establish radio links between mobile subscribers and mobile switching centers. Each mobile subscriber is permanently assigned a unique telephone number recognized by the public switched network. Thus, calls to a particular mobile subscriber are routed to an interrogation exchange which consults the home exchange of that particular subscriber to obtain information regarding where the call should be routed in order to reach the mobile subscriber. If the mobile subscriber cannot be reached at the location to which the call was routed the call is lost and the resources of the communications network have been occupied unnecessarily.
Although there are no known prior art teachings of a solution to the aforementioned deficiencies and shortcomings of prior art mobile communications systems, a number of prior art references exist that discuss subject matter that relates to the subject matter of the present application. Exemplary of such prior art references are U.S. Pat. Nos. 4,228,319, 4,612,415, 4,748,655, 4,794,635, 4,833,701, and 4,833,702. Each of these references is discussed briefly below.
U.S. Patent No. 4,228,319 to De Jager et al. relates to an automatic mobile subscriber system. De Jager et al. teach stationary stations which repeatedly transmit through their cell channel a channel number of a free duplex speech channel which is stored in memory in mobile stations receiving the call channel. De Jager et al. also teach use of the stored channel number for automatically selecting the free duplex speech channel in response to a connection set-up command.
U.S. Pat. No. 4,612,415 to Zdunek et al. relates to a method and means for controlling telephone interconnect traffic in a trunked radio system. Zdunek et al. teach continuously monitoring all types of communication traffic on the system and, in response to an increasing dispatch access delay, reserving certain repeaters for dispatch use only during a predetermined period. Zdunek et al. also teach a system in which the number of simultaneous telephone interconnect calls permitted on the system during the predetermined period is dynamically altered in response to system loading. Still further, Zdunek et al. teach establishment of a variable, periodically updated, maximum interconnect call length based on the current system dispatch access delay.
U.S. Pat. No. 4,748,655 to Thrower et al. relates to portable telephones. According to the teachings of Thrower et al., a personal portable telephone allows a user access to a cellular radio network via a number of different gateways including a communal unit, a user's mobile telephone set, a home telephone or an office PABX. The telephone can also be used as a paging unit for use within the cellular radio network. The communal radio unit is a multi-channel arrangement capable of servicing a large number of personal telephones within its service area which is small compared to that of a normal cellular radio cell. The communal units may be transportable and located in public -areas such as railway stations, airports and on trains or coaches. The telephone automatically transmits its identification number to a gateway when it enters its service area enabling the subscriber to be accessed by callers without knowing his whereabouts.
U.S. Pat. No. 4,794,635 to Hess teaches a two-way radio communication system having a limited number of channels which assigns calls according to a max-minimum method. Hess' method also ascertains the type of call, handoff or first assignment, and assigns a server with a given central controller to a channel frequency within the requestor sector that minimizes interference to present users. In a second embodiment of his method, Hess teaches requesting assignment to a limited number of channels to reduce adjacent channel interference to and from the requestor with respect to present users operating on a system by utilizing one or more thresholds to arrange channel assignments based upon progressively larger or smaller values of received signal strength.
U.S. Pat. No. 4,833,701 to Comroe et al. relates to a trunked communications system with nationwide roaming capability. According to the teachings of Comroe et al., selected regional trunking systems are equipped with telephone interchange capability and provided with local computers, which communicate with a national hub computer. At each selected trunking system, several ID's are reserved as "roaming ID's" to be temporarily assigned to roaming subscribers. When a subscriber determines that he has roamed into a new trunk system, he requests a roaming ID. A roaming ID is assigned and transmitted to the subscriber, which thereafter operates within the new trunked system using its roaming ID. The roaming assignment is also transmitted to the national hub computer so that interconnect calls may be properly forwarded. The roaming subscriber continues to operate under the assigned roaming ID until it roams out of range of the current system and into yet another trunked system In this way, the subscriber may roam from system to system.
U.S. Pat. No. 4,833,702 to Shitara et al. relates to telephone registration and cancellation control in a wide area cordless telephone system. Specifically, Skitara et al. teach a method for making a registration of a roaming cordless telephone in one of a plurality of local service areas and cancelling the registration.